USE OF SBR LATEXES TO MITIGATE INFERIOR CONCRETE ......USE OF SBR LATEXES TO MITIGATE INFERIOR CONCRETE PROPERTIES RESULTING FROM RECYCLED COARSE AGGREGATES Yehia Daou Professor and
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variations in bond stresses. This paper can be of interest to environmental
organizations and concrete engineers dealing with composite structures and efficient
re-use of RCA materials in the construction industry.
2. EXPERIMENTAL PROGRAM
2.1. Coarse aggregate characterization
Two RCAs having 20-mm nominal size were used; the RCA1 was obtained by
crushing returned concrete from ready-mixed batching plant, while RCA2 consisted
of crushed concrete recuperated from processing old infrastructure elements such as
manholes, concrete pipes, and culverts. Also, continuously graded crushed limestone
NCA having 20-mm nominal size was employed. The aggregates gradations were
within ASTM C33 limitations, sieve No. 67 [13].
The physical NCA and RCA properties are summarized in Table 1. The freeze-
thaw test procedure was used to determine the adhered mortar portion of RCA. The
materials were immersed in sodium sulphate solution, and subjected to five daily
cycles of freezing and thawing. After the final cycle, the sodium sulphate solution was
drained and aggregates washed and sieved over a 4.75-mm sieve. The aggregate
crushing value (ACV), reflecting the compressive strength of loose aggregate, was
determined by subjecting a measured volume of aggregate to 400-kN load [14]. After
crushing, the sample is sieved over 2.36-mm sieve where the percentage of material
passing the sieve represents the ACV (i.e., higher ACV value reflects weaker
aggregates with lower compressive strength).
2.2. Materials used for concrete production and bond testing
Portland cement conforming to ASTM C150 Type I was used; its surface area,
median particle size, and specific gravity were 355 m2/kg, 24.7 m, and 3.14,
respectively. The natural fine aggregate consisted of well-graded siliceous sand
complying to ASTM C33 specification [13]; its bulk specific gravity, fineness
modulus, and absorption rate were 2.65, 2.5, and 0.97%, respectively. A naphthalene-
based high-range water-reducing (HRWR) admixture with specific gravity of 1.18
and solid content of 34% was used. This admixture complies with ASTM C494 Type
F; it can be used up to 3.5% of cement mass.
Table 1 Properties of NCA and RCAs used for concrete batching
Specific
gravity
Oven-dry
rodded bulk
density, kg/m3
Absorption
rate, %
Material
finer than
75-m, %
Fineness
modulus
Adhered
mortar
content, %
ACV,
%
NCA 2.72 1,763 0.61 0.42 6.71 n/a 17.8
RCA1 2.43 1,505 7.04 0.9 6.77 41.2 23.1
RCA2 2.4 1,497 6.12 1.16 6.84 44 28.2
Commercially available white SBR latex typically used for enhancing flexibility
and water-impermeability of cementitious materials was used. The carboxylated
styrene butadiene dispersion contains 60% of bound styrene without solvents and
stabilized using anionic emulsifying system. Its solid content, specific gravity, pH,
Brookfield viscosity (spindle 4 at 10 rpm), maximum particle size, and minimum film
forming temperature (MFFT) are 56%, 1.05, 8.5, 250 cP, 0.22 m, and -5 °C, respectively. Relevant research studies examining the effect of SBR latexes on fresh
2.4. Specimen preparation and experimental testing
Part 1-Effect of SBR concentration on RCA1 and RCA2 concrete strength
Following the end of mixing, the slump, unit weight, and air content were determined
as per ASTM C143, C138, and C231 Test Methods, respectively. The fresh concrete
mixtures were then filled in 100×200 mm steel cylinders and covered by wet burlap
for 24 hours. After demolding, all concrete specimens prepared without SBR were
placed in a moist-curing room at 23 3 °C and more than 95% relative humidity (RH). For mixtures containing SBR, the specimens were divided into two groups: the first
was wet-cured in similar manner as earlier described, while the second group was
cured for 24 hours in 95% RH, followed by air curing at 23 3 °C at 50% 5% RH, as
per ACI 548 recommendations [16]. By subjecting the specimens to various curing
conditions, the intention was to capture the effect of SBR on strength variations under
different curing conditions.
At 28 days, the compressive strength (f’c) and splitting tensile strength (ft) were
determined as per ASTM C39 and C496 Test Methods, respectively. Averages of 3
measurements are considered in this paper; the failure planes of concrete cylinders
were examined visually after crushing using a magnifying glass and classified as
being mainly around or mainly through the aggregate skeleton [8].
Part 2-Effect of SBR on bond stress vs. slip behavior
The effect of SBR additions on bond to steel behavior was determined using beam-
end specimens, as per ASTM A944 Test Method (only the RCA1 was considered in
this part). The specimen dimensions measured 220 mm in width, 250 mm in length,
and 220 mm in height, as shown in Fig. 1 [17]. Deformed steel bars complying to
ASTM A615 No. 13 with nominal diameter (db) of 12.7 mm was used; the Young’s
modulus and yield strength (fy) were 203 GPa and 428 MPa, respectively. The
specimen is positioned in a test rig so that the bar can be pulled slowly from concrete;
it is restrained from translation through a compression reaction plate and restrained
from rotation through a tie-down, thus approximating boundary conditions of simply
supported beams.
All bars were embedded inside the specimens at fixed lengths of 5db (i.e., 60 mm)
to prevent yielding of steel; similar rib orientation with respect to pullout load was
maintained in all specimens. The concrete clear cover was kept constant at 40 mm; a
size typically used in the design of flexural beams. Two stirrups with 9.5-mm nominal
diameter were placed on each side were provided for shear resistance, but were
oriented parallel to the “pull” direction to avoid confining the test bar along its bonded
length. The concrete samples were placed in two consecutive lifts in the beam-end
specimen molds, and internally vibrated using 150-Hz frequency vibrator. Before
testing, the specimen was shimmed and aligned so that the test bar is parallel to the
loading frame. The tensile load was gradually applied at a rate of 25 ±4 kN per minute
until bond failure occurred. The bar’s relative slips to concrete were monitored from
measurements of two LVDTs placed at the free and loaded bar surfaces (Fig. 1).
Figure 2 Effect of curing regime on f’c and ft of SBR-modified RCA concrete
3.1.3 Effect of SBR on variations in f’c and ft
The variations in hardened properties for concrete mixtures containing RCA1 with or
without SBR are plotted in Fig. 3. The (Property) was normalized with respect to corresponding control NCA concrete made with either 320 or 440 kg/m
3 cement, as
follows:
A. Compressive strength
Concrete made without polymers – As can be seen, the complete NCA substitution by
RCA1 led to reduced f’c, particularly for high strength mixtures prepared with 440
kg/m3 cement. Hence, this varied from -5.7% to -15.3% for concrete made with 320
or 440 kg/m3 cement, respectively. Such results are in agreement with those reported
by Butler et al. [8] who associated the drop in f’c of lean and high strength RCA
concrete to different failure planes occurring around or through the coarse aggregate
skeleton. In fact, the visual examination of crushed concrete cylinders made with 320
kg/m3 cement showed distinct failure planes occurring mainly around the aggregate
particles, suggesting that the ITZ between mortar-aggregate is the limiting strength
factor. In contrast, the failure planes become less distinct and mostly passing through
the aggregate particles for concrete prepared with 440 kg/m3, implying that the
strength of RCA itself is the limiting factor [1,8].
Effect of SBR on (f’c) – Clearly, the use of increased SBR concentration improved f’c of RCA1 mixtures, albeit this varied depending on cement content and
strength of original concrete (Fig. 3). For example, (f’c) increased significantly from
-5.7% for lean 320-RCA1 concrete made without SBR to +7.3% and +8.2% for
equivalent mixtures containing 2% or 3% SBR, respectively. This can be attributed to
the polymer particles that strengthen the mortar-aggregate interface, especially
knowing that f’c of lean concrete is mostly governed by the ITZ behavior. In contrast,
the (f’c) increase in high strength concrete prepared with 440 kg/m3 cement was
Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled
much less pronounced, and remained in the negative region. Hence, (f’c) varied from
-15.3% for 440-RCA1 concrete made without SBR to -9.7% and -3.3% for mixtures
containing 3% and 4% SBR, respectively. This practically suggests that the beneficial polymer effect on compressive strength of recycled aggregate concrete is directly
affected by the mixture proportioning.
Figure 3 Effect of SBR on variations in f’c and ft for concrete containing RCA1
B. Splitting tensile strength
Unlike (f’c), the incorporation of SBR polymers led to gradually increased ft values,
even much higher than corresponding NCA concrete. For example, (ft) reached
+14.1% and +8.9% for RCA1 mixtures made with 320 or 440 kg/m3 cement,
respectively, containing 3% SBR. In fact, it is well accepted that structure of hardened
cement paste is mainly composed of agglomerated calcium silicate hydrates and
calcium hydroxide bound together by weak van der Waals forces, whereby
microcracks could occur easily under stress leading to poor tensile strength [9].
Hence, in latex-modified systems, the microcracks are bridged by the polymer films
that prevent crack propagation in hardened ITZ, resulting in stronger cement hydrate-
aggregate bond. Additionally, the improved smoothness and flowability of modified
RCA concrete are expected to reduce porosity of ITZ and develop increased bond by
micro-mechanical interlocking mechanisms [11].
3.1.4 Effect of p/c and comparison between RCA1 vs. RCA2
The relationships between p/c with respect to (f’c) and (ft) determined on concrete mixtures prepared using RCA1 and RCA2 cured under air conditions are plotted in
Fig. 4. Clearly, the incorporation of SBR (i.e., higher p/c) led increased strength
development, albeit this appears to be significantly affected by the type of RCA used.
As can be seen, the tendency curves obtained from concrete containing RCA1 are
consistently higher than those resulting from RCA2 concrete. This could be directly
related to the quality of recycled aggregates (i.e., note that RCA1 was of better quality
than RCA2, the resulting ACV was 23.1% vs. 28.2%, respectively). Hence, the
threshold p/c beyond which f’c becomes equivalent to NCA mixture decreased from
around 2% to 1.2% with the use of RCA2 and RCA1, respectively. For ft, such
decrease was from around 1.4% to 0.9%, respectively. Practically speaking, this
clearly shows the importance of RCA quality on the development of concrete
strength.
3.2. Phase II: Effect of SBR additions on bond stress vs. slip behavior
Table 4 summarizes the bond characteristics of tested concrete including the bond
stresses corresponding to slip of 0.01 and 0.1 mm (0.01mm and 0.1mm, respectively),
ultimate bond stress (u) representing the maximum load at failure, and slip at free-
end (δu) coinciding with the ultimate load. Also, the normalized bond stress calculated
as the ratio of u to the square root of f’c is given. It is to be noted that all tests exhibited pullout modes of failure characterized by crushing and shearing of the
localized embedded region around the bar. No cracks were observed on their external
surfaces, indicating that the concrete cover provided adequate confinement [17].
3.2.1. Bond stress vs. slip curves of tested mixtures
The vs. δ curves determined for control NCA concrete prepared with 320 kg/m3
cement as well as those made using RCA1 with or without SBR additions are given in
Fig. 5.
Comparison between NCA vs. RCA1 concrete behavior (without SBR) –
Concurrent with existing literature [8,9,10,12], the substitution of NCA by RCA1 did
not result in considerable changes in τ vs. δ curves. Hence, the three mechanisms
controlling the bond between steel and concrete including adhesion, mechanical
interlock, and friction can be well identified [7]. Nevertheless, it is to be noted that u at failure for RCA1 concrete was relatively lower than equivalent value determined
using NCA mixture, especially for higher strength concrete prepared with 440 kg/m3
cement. For example, u decreased from 11.8 to 11.4 MPa and from 16.3 to 14.7 MPa
for mixtures made with 320 and 440 kg/m3 cement, respectively (Table 4). This can
be directly attributed to the reduced RCA1 concrete hardened properties including f’c
and ft, thus reducing the material’s bearing strength capacity in front of the bar ribs.
Figure 4 Effect of p/c and RCA1 vs. RCA2 on variations in f’c and ft
y=13.79x-7.07R²=0.69
y=20.88x-22.36R²=0.84
-30
-20
-10
0
10
20
30
Δ(),%
RCA1(ACV=23.1%)
RCA2(ACV=28.2%)
Dataobtainedunderair-curingregime
y=6.81x-9.11R²=0.26
y=12.35x-23.93R²=0.75
-30
-20
-10
0
10
20
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1
Δ(f'c),%
Polymer/Cementra o(p/c),%
Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled
Table 4 Effect of SBR on bond stress vs. slip concrete properties
Mixture codification 0.01mm,
MPa
0.1mm,
MPa
u,
MPa
δu,
mm u / (f’c)
0.5
320-NCA 2.54 7.13 11.8 0.53 2.1
320-RCA1 2.3 6.65 11.4 0.55 2.09
320-RCA1-1%SBR 2.7 7.45 12.8 0.71 2.29
320-RCA1-2%SBR 3.05 9 13 0.9 2.23
320-RCA1-3%SBR 4.6 8.77 14.1 0.88 2.41
440-NCA 3.85 9.6 16.3 0.48 2.2
440-RCA1 3.24 7.8 14.7 0.48 2.16
440-RCA1-2%SBR 5.3 12.7 17.1 0.92 2.47
440-RCA1-3%SBR 7.2 14 18.7 0.9 2.66
440-RCA1-4%SBR 9.7 16.1 21.4 1.32 2.94
Figure 5 Typical bond stress vs. slip curves for mixtures made with 320 kg/m3 cement
Behavior of RCA1 concrete containing SBR – Clearly, the bars free-end of
polymer-modified RCA1 concrete started to slip at bond stresses higher than those of
control mixtures, thus accentuating the initial stiffness of vs. δ curves (Fig. 5). For
example, at the very small slip of 0.01 mm, 0.01mm increased from 2.3 MPa for 320-RCA1 concrete prepared without SBR to 3.05 and 4.6 MPa with the addition of 2% or
3% SBR, respectively (Table 4). This can be directly attributed to the latex polymers
that increase the adhesive component in the elastic region and result in increased
interfacial shear stresses between the reinforcing bar and surrounding concrete [12].
Ohama [9] related this phenomenon to the presence of electro-chemically active
polymer-cement co-matrixes at the interfaces with reinforcing bars, thus relaxing the
stresses during loading and retarding the friction-controlled slip of rebars.
When the adhesive component of bond fails, the responses of ascending curves of
SBR-modified concrete showed extended non-linear regions together with higher u,
which can be explained by more pronounced compressive strain-softening
phenomenon due to the presence of polymer latexes [7]. For example, δu increased
from 0.55 mm for 320-RCA1 concrete prepared without SBR to 0.88 mm with 3%
SBR; the corresponding u increased from 11.4 MPa to 13 MPa (Table 4). In fact, the bond resistance in beam-end specimen is achieved by circumferential tension stresses
created in the concrete around the bar; if these forces exceed the tensile concrete
capacity, failure occurs [7,8,12]. Therefore, given that ft of RCA1 concrete is
significantly improved with polymer additions, this can reduce the propagation of
microcracks and result in increased bond resistance with reinforcing bar.
3.2.2. Relationships between p/c and bond properties
The relationships between p/c with respect to 0.01mm, 0.1mm, and u are plotted in Fig. 6. As can be seen, RCA1 concrete incorporating higher SBR additions (i.e., higher
p/c) led to increased bond stresses. Nevertheless, such increase was particularly
accentuated for 0.01mm, suggesting that the adhesive component of bond could be
highly improved by such additions. For example, at the highest p/c of 2.06%,
(0.01mm) reached 152%, while (0.1mm) and (u) reached respectively 68% and 31%.
From the other hand, it is to be noted that the slips at failure shifted gradually
towards higher values with increased p/c. The resulting correlation can be written as:
Slip at free-end, mm = 0.309 (p/c, %) + 0.51, with R2 of 0.88. Practically, this
indicates that the structural ductility of reinforced RCA concrete members tends to
increase with polymer additions [12]. The ratio of u to the square root of f’c followed
an increasing trend with p/c; the relationship can be written as: Ratio = 0.296 (p/c, %)
+ 2.11, with R2 of 0.78.
Figure 6 Relationships between p/c with respect to bond stresses
4. SUMMARY AND CONCLUSIONS
The use of RCA in structural concrete applications is very limited, given the concerns
pertaining to its inferior mechanical properties when compared to natural aggregate
concrete. The main objective of this paper is to evaluate the effect of SBR polymers
on RCA concrete properties and, consequently, bond to embedded steel bars.
Based on foregoing, test results have shown that the incorporation of SBR can
remarkably improve RCA concrete workability due to ball bearing and plasticizing
effects. On the hardened state, such additions led to increased f’c and ft, particularly
for lean mixtures made with 320 kg/m3 cement, which practically implies that
polymeric latexes can effectively compensate the loss in RCA concrete performance.
The improvement in strength was further accentuated when curing was realized for 24
y=2.38x2-1.93x+3.01(R²=0.83)
y=1.82x2-0.22x+7.77(R²=0.66)
y=2.83x2-2.48x+13.57(R²=0.58)
2
6
10
14
18
22
0 0.5 1 1.5 2 2.5
Bondstress,M
Pa
Polymer/Cementra o(p/c),%
τ0.01mm
τ0.1mm
τu
Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled
hours in 95% RH, and then for 27 days in air conditions at 23 3 °C and 50% 5%
RH.
The mechanisms of bond failure in vs. δ curves recorded using SBR-modified RCA concrete are fundamentally similar to those observed with natural aggregate
concrete. Yet, the initial stiffness was considerably accentuated with SBR additions,
reflecting increased interfacial shear stresses between the reinforcing bar and
surrounding concrete. Also, the ascending curves showed extended non-linear regions
together with higher u. Good correlations were established between p/c and bond properties.
ACKNOWLEDGMENTS
This project was funded by the School of Engineering Research Council of the
Lebanese University (LU), Hadath, Lebanon. The authors wish to acknowledge the
experimental support provided by the Laboratory of the Civil Engineering Department
at LU as well as the contributions of research assistants from Finders SAL, Amchit,
Lebanon.
REFERENCES
[1] Rilem TC 217. Progress in recycling in the built environment. Springer
publication, Ed. E. Vanquez. ISBN 978-94-007-4907-8, (2013), 400 p.
[2] ACI 555R-01. Removal and reuse of hardened concrete. ACI Committee 555,
(2001), 26 p.
[3] Rao, G.A., Prasad B.K.R. Influence of the roughness of aggregate surface on the
interface bond strength, Cement and Concrete Research 32, (2002), 253-257.
[4] Hansen, T.C., and Narud, H. (1983). Strength of recycled concrete made from